Network File System Version 4 C. Lever
Internet-Draft Oracle
Intended status: Informational November 6, 2018
Expires: May 10, 2019
RDMA Connection Manager Private Data For RPC-Over-RDMA Version 1draft-ietf-nfsv4-rpcrdma-cm-pvt-data-01
Abstract
This document specifies the format of RDMA-CM Private Data exchanged
between RPC-over-RDMA version 1 peers as a transport connection is
established. Such private data is used to indicate peer support for
remote invalidation and larger-than-default inline thresholds.
Status of This Memo
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This Internet-Draft will expire on May 10, 2019.
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Internet-Draft RPC-Over-RDMA CM Private Data November 2018
This document specifies a simple, non-XDR-based message format
designed to be passed between RPC-over-RDMA version 1 peers at the
time each RDMA transport connection is first established. The
purpose of this message format is two-fold:
o To provide immediate relief from certain performance constraints
inherent in RPC-over-RDMA version 1
o To enable experimentation with parameters of the base RDMA
transport over which RPC-over-RDMA runs
The message format may be extended as needed. In addition,
interoperation between implementations of RPC-over-RDMA version 1
that present this message format to peers and those that do not
recognize this message format is guaranteed.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Advertised Transport Properties3.1. Inline Threshold SizeSection 3.3.2 of [RFC8166] defines the term "inline threshold." An
inline threshold is the maximum number of bytes that can be
transmitted using one RDMA Send and one RDMA Receive. There are a
pair of inline thresholds for a connection: a client-to-server
threshold and a server-to-client threshold.
If an incoming message exceeds the size of a receiver's inline
threshold, the receive operation fails and the connection is
typically terminated. To convey a message larger than a receiver's
inline threshold, an NFS client uses explicit RDMA data transfer
operations, which are more expensive to use than RDMA Send.
The default value of inline thresholds for RPC-over-RDMA version 1
connections is 1024 bytes (see Section 3.3.3 of [RFC8166]). This
value is adequate for nearly all NFS version 3 procedures.
NFS version 4 COMPOUND operations [RFC7530] are larger on average
than NFS version 3 procedures [RFC1813], forcing clients to use
explicit RDMA operations for frequently-issued requests such as
LOOKUP and GETATTR. The use of RPCSEC_GSS security also increases
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the average size of RPC messages, due to the larger size of
RPCSEC_GSS credential material included in RPC headers [RFC7861].
If a sender and receiver could somehow agree on larger inline
thresholds, frequently-used RPC transactions avoid the cost of
explicit RDMA operations.
3.2. Remote Invalidation
After an RDMA data transfer operation completes, an RDMA peer can use
remote invalidation to request that the remote peer RNIC invalidate
an STag associated with the data transfer [RFC5042].
An RDMA consumer requests remote invalidation by posting an RDMA Send
With Invalidate Work Request in place of an RDMA Send Work Request.
Each RDMA Send With Invalidate carries one STag to invalidate. The
receiver of an RDMA Send With Invalidate performs the requested
invalidation and then reports that invalidation as part of the
completion of a waiting Receive Work Request.
An RPC-over-RDMA responder can use remote invalidation when replying
to an RPC request that provided Read or Write chunks. The requester
thus avoids dispatching an extra Work Request, the resulting context
switch, and the invalidation completion interrupt as part of
completing an RPC transaction that uses chunks. The upshot is faster
completion of RPC transactions that involve RDMA data transfer.
There are some important caveats which contraindicate the blanket use
of remote invalidation:
o Remote invalidation is not supported by all RNICs.
o Not all RPC-over-RDMA responder implementations can generate RDMA
Send With Invalidate Work Requests.
o Not all RPC-over-RDMA requester implementations can recognize when
remote invalidation has occurred.
o On one connection in different RPC-over-RDMA transactions, or in a
single RPC-over-RDMA transaction, an RPC-over-RDMA requester can
expose a mixture of STags that may be invalidated remotely and
some that must not be. No indication is provided at the RDMA
layer as to which is which.
A responder therefore must not employ remote invalidation unless it
is aware of support for it in its own RDMA stack, and on the
requester. And, without altering the XDR structure of RPC-over-RDMA
version 1 messages, it is not possible to support remote invalidation
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with requesters that mix STags that may and must not be invalidated
remotely in a single RPC or on the same connection.
However, it is possible to provide a simple signaling mechanism for a
requester to indicate it can deal with remote invalidation of any
STag it has presented to a responder. There are some NFS/RDMA client
implementations that can successfully make use of such a signaling
mechanism.
4. Private Data Message Format
With an InfiniBand lower layer, for example, RDMA connection setup
uses a Connection Manager when establishing a Reliable Connection
[IBARCH]. When an RPC-over-RDMA version 1 transport connection is
established, the client (which actively establishes connections) and
the server (which passively accepts connections) populate the CM
Private Data field exchanged as part of CM connection establishment.
The transport properties exchanged via this mechanism are fixed for
the life of the connection. Each new connection presents an
opportunity for a fresh exchange.
For RPC-over-RDMA version 1, the CM Private Data field is formatted
as described in the following subsection. RPC clients and servers
use the same format. If the capacity of the Private Data field is
too small to contain this message format, the underlying RDMA
transport is not managed by a Connection Manager, or the underlying
RDMA transport uses Private Data for its own purposes, the CM Private
Data field cannot be used on behalf of RPC-over-RDMA version 1.
The first 8 octets of the CM Private Data field is to be formatted as
follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Format Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version | Flags | Send Size | Receive Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Format Identifier: This field contains a fixed 32-bit value that
identifies the content of the Private Data field as an RPC-over-
RDMA version 1 CM Private Data message. The value of this field
is always 0xf6ab0e18, in network byte order. The use of this
field is further expanded upon in Section 4.1.
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Version: This 8-bit field contains a message format version number.
The value "1" in this field indicates that exactly eight octets
are present, that they appear in the order described in this
section, and that each has the meaning defined in this section.
Further considerations about the use of this field are discussed
in Section 5.
Flags: This 8-bit field contains bit flags that indicate the support
status of optional features, such as remote invalidation. The
meaning of these flags is defined in Section 5.1.
Send Size: This 8-bit field contains an encoded value corresponding
to the maximum number of bytes this peer is prepared to transmit
in a single RDMA Send on this connection. The value is encoded as
described in Section 5.2.
Receive Size: This 8-bit field contains an encoded value
corresponding to the maximum number of bytes this peer is prepared
to receive with a single RDMA Receive on this connection. The
value is encoded as described in Section 5.2.
4.1. Interoperability Considerations
The extension described in this document is designed to allow RPC-
over-RDMA version implementations that use this extension to
interoperate fully with RPC-over-RDMA version 1 implementations that
do not exchange this information. Realizing this goal requires that
implementations of this extension follow the practices described in
the rest of this section.
RPC-over-RDMA version 1 implementations that support the extension
described in this document are intended to interoperate fully with
RPC-over-RDMA version 1 implementations that do not recognize the
exchange of CM Private Data. When a peer does not receive a CM
Private Data message which conforms to Section 4, it needs to act as
if the remote peer supports only the default RPC-over-RDMA version 1
settings, as defined in [RFC8166]. In other words, the peer is to
behave as if a Private Data message was received in which bit 8 of
the Flags field is zero, and both Size fields contain the value zero.
The Format Identifier field is provided in order to distinguish RPC-
over-RDMA version 1 Private Data from private data inserted by layers
below or above RPC-over RDMA version 1. During connection
establishment, RPC-over-RDMA version 1 implementations check for this
protocol number before decoding subsequent fields. If this protocol
number is not present as the first 4 octets, an RPC-over-RDMA
receiver needs to ignore the CM-Private Data (ie., behave as if no
RPC-over-RDMA version 1 Private Data has been provided).
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Internet-Draft RPC-Over-RDMA CM Private Data November 20185. Updating the Message Format
Although the message format described in this document provides the
ability for the client and server to exchange particular information
about the local RPC-over-RDMA implementation, it is possible that
there will be a future need to exchange additional properties. This
would make it necessary to extend or otherwise modify the format
described in this document.
Any modification faces the problem of interoperating properly with
implementations of RPC-over-RDMA version 1 that are unaware of this
existence of the new format. These include implementations that that
do not recognize the exchange of CM Private Data as well as those
that recognize only the format described in this document.
Given the message format described in this document, these
interoperability constraints could be met by the following sorts of
new message formats:
o A format which uses a different value for the first four bytes of
the format, as provided for in the registry described in
Section 6.
o A format which uses the same value for the Format Identifier field
and a value other than one (1) in the Version field.
Although it is possible to reorganize the last three of the eight
bytes in the existing format, extended formats are unlikely to do so.
New formats would take the form of extensions of the format described
in this document with added fields starting at byte eight of the
format and changes to the definition of previously reserved flags.
5.1. Feature Support Flags
The bits in the Flags field are labeled from bit 8 to bit 15, as
shown in the diagram above. When the Version field contains the
value "1", the bits in the Flags field are to be set as follows:
Bit 15: When both connection peers have set this flag in their CM
Private Data, the responder MAY use RDMA Send With Invalidate when
transmitting RPC Replies. Each RDMA Send With Invalidate MUST
invalidate an STag associated only with the XID in the rdma_xid
field of the RPC-over-RDMA Transport Header it carries.
When either peer on a connection clears this flag, the responder
MUST use only RDMA Send when transmitting RPC Replies.
Bits 14 - 8: These bits are reserved and are always zero.
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Internet-Draft RPC-Over-RDMA CM Private Data November 20185.2. Inline Threshold Values
Inline threshold sizes from 1KB to 256KB can be represented in the
Send Size and Receive Size fields. A sender computes the encoded
value by dividing the actual value by 1024 and subtracting one from
the result. A receiver decodes this value by performing a
complementary set of operations.
The client uses the smaller of its own send size and the server's
reported receive size as the client-to-server inline threshold. The
server uses the smaller of its own send size and the clients's
reported receive size as the server-to-client inline threshold.
6. IANA Considerations
In accordance with [RFC8126], the author requests that IANA create a
new registry in the "Remote Direct Data Placement" Protocol Category
Group. The new registry is to be called the "RDMA-CM Private Data
Identifier Registry". This is a registry of 32-bit numbers that
identify the Upper Layer protocol associated with data that appears
in the RDMA-CM Private Data area.
The information that must be provided to add an entry to this
registry will be an IESG-approved Standards Track specification
defining the semantics and interoperability requirements of the
proposed new value and the fields to be recorded in the registry.
The fields in this registry include: Field Identifier, Format
Description, and Reference.
The initial contents of this registry are a single entry:
+-----------------+-------------------------------------+-----------+
| Field | Format Description | Reference |
| Identifier | | |
+-----------------+-------------------------------------+-----------+
| 0xf6ab0e18 | RPC-over-RDMA version 1 CM Private | [RFC-TBD] |
| | Data | |
+-----------------+-------------------------------------+-----------+
Table 1: RDMA-CM Private Data Identifier Registry
The Expert Review policy, as defined in Section 4.5 of [RFC8126] is
to be used to handle requests to add new entries to the "File
Provenance Information Registry". New protocol numbers can be
assigned at random as long as they do not conflict with existing
entries in this registry.
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